The Philippine coast guard spotted about 220 Chinese vessels moored at the reef, which Beijing and Vietnam also claim, on March 7.
On Monday, a surveillance aircraft spotted 183 Chinese vessels still at the reef, said Philippine military chief Lt. Gen. Cirilito Sobejana, who released aerial pictures of the Chinese vessels in one of the most hotly contested regions in the strategic waterway.
The Philippines has filed a diplomatic protest over the Chinese presence, Foreign Secretary Teodoro Locsin Jr. said.
China insisted it owns the reef, which it calls Niué Jiao, and said the Chinese vessels converged in the area to avoid rough waters.